Throughout the trial, which lasted almost four weeks, the main defendant, Daoudi Yahiaoui, 50, minimized his role in the 2008 robbery in the so-called golden triangle of luxury boutiques in Paris. But the tribunal concluded that he was the brain behind the robbery and a holdup in 2007, which together resulted in the theft of more than 900 diamonds and other jewels.

Mr. Yahiaoui was one of eight men on trial, and the others were sentenced to prison terms of nine months to 15 years. With video of the 2008 robbery, the plot unraveled over the years as the thieves sought to sell the gems, unaware that their telephones were tapped and they were being tailed by investigators seeking to recover the jewels. About 500 gems remain missing.

Mouloud Djennad, 39, the guard who admitted being the “inside man” in the plot, was sentenced to five years, but he remained free with three years of the punishment suspended and additional time reduced for the period he spent in pretrial detention.

During the trial, he expressed remorse for his actions, apologizing to a former co-worker, who was a witness, and weeping in his hands.

The trial, which started Feb. 3, offered an unusual view of the plotting of a diamond theft, one of many that jolted the luxury districts of the French capital in a wave of organized robberies.

The robbery seemed flawlessly executed, but the thieves were tripped up by a series of mistakes, like leaving behind a handbag with a fingerprint, and by their troubled efforts to sell the diamonds amid tensions with intermediaries scouting for buyers.

In March 2011, on the fifth search of Mr. Yahiaoui’s home in a Paris suburb, investigators found stuffed in a drainpipe a hand cream bottle that contained missing earrings, assorted rings and a 31-carat diamond solitaire.

The actual estimates of the loss varied because of the difference in retail and wholesale values of the gems. In court, a lawyer for Harry Winston testified that the company had received insurance payments of $36.7 million for the 2007 holdup and $52.6 million for the second robbery.

Many of the men on trial are related to each other. Mr. Yahiaoui’s brother-in-law introduced the guard to him after he casually revealed security weaknesses in the jewelry store. Mr. Yahiaoui’s brother and nephew were also sentenced in the plot.

Lawyers for the guard defended his involvement by saying he was naïve and trusted Mr. Yahiaoui because they shared family roots in the Kabylie region in northeastern Algeria. Mr. Djennad, who has been working in a butcher shop while awaiting the trial, said he had become trapped in his friendship with Mr. Yahiaoui, who intimidated him into participating in the second robbery in 2008 despite his misgivings after the first holdup.

Grainy black-and-white video from the store shows him opening the door to the men in their outlandish outfits and then standing by the door as they fled with a rolling suitcase of gems.

A version of this article appears in print on , Section A, Page 6 of the New York edition with the headline: Leader of Paris Jewel Heist Is Given 15-Year Prison Term. Order Reprints | Today’s Paper | Subscribe